Remediation refers to the treatment of geological formations to improve the recovery of hydrocarbons from well damage and arterial blockage caused by the precipitation and deposition of heavy organic molecules from petroleum fluids. Heavy organic molecules such as asphaltenes, asphaltogenic acids, diamondoids and derivatives, mercaptans, organometallics, paraffin waxes and resins exist in crude oils in various quantities and forms. Such compounds could separate out of the crude oil solution due to various mechanisms and deposit, causing fouling in the oil reservoir, in the well, in the pipelines and in the oil production and processing facilities. Solid particles suspended in the crude oil may stick to the walls of the conduits and reservoirs reducing oil production from the wells. Well damage caused by the precipitation and deposition of paraffin and asphaltenes have been a recurrent problem in the production of crude oil and can be caused by a number of standard oilfield operations. The addition of large volumes of cold fluids during acidizing and fracturing operations in low bottom hole temperature wells can cause an irreversible process of paraffin deposition and subsequent formation damage. The standard practice of hot oiling producing wells for the removal of paraffin from the tubing producing wells for the removal of paraffin from the tubing string is another source of potential damage to a well's producing capacity. Asphaltene deposition in the formation and well bore area results from the use of hydrochloric acid during acidizing, addition of low surface tension organic liquids such as diesel, kerosene, or gasoline and the use of CO2 injection for EOR projects. Paraffin deposits consist of a mixture of linear and branched chained hydrocarbons in the range of C18H38 to C60H122, generally mixed with other organic and inorganic materials such as crude oil, gums, resins, asphaltic material, salt, sand, clay and water. The accumulation of paraffin wax on the formation face and the near well bore area of the formations leads to a decrease in permeability and production of crude oil. Paraffin precipitates from the crude oil at an equilibrium temperature and pressure defined as the cloud point.
Chemical control of these problems relies upon the use of 4 categories of chemicals. Solvents are generally used to dissolve existing deposits and usually contain a high aromatic content. They dissolve a specific weight of paraffin based upon the molecular weight of the wax, temperature, and pressure before the solvent power is exhausted.
Dispersants do not dissolve paraffin deposits but rather break them up into much smaller particle sizes where they can be reabsorbed by the oil stream. Dispersants may diffuse several times their own weight in paraffin but do not have the widespread application of solvents. Generally, given the proper testing techniques a chosen dispersant will prove to be more cost-effective than solvents.
Detergents are a class of surface active agents that work in the presence of water to water-wet paraffin particles, formation, tubing and flowlines. These agents break up deposits and prevent them from reagglomerating back together further downstream in the system.
Stimulation refers to the removal of unwanted deposits from the wellbore and production equipment. Remediation includes hydrogen sulfide mitigation. Such unwanted deposits form and/or accumulate in the wellbore, production and recovery equipment and well casing. Under static reservoir conditions, asphaltenes are normally held in a stable suspension by resins, a family of polar molecules. However, changes in fluid temperature and pressure that are associated with oil production from the reservoir may cause the asphaltenes to flocculate and precipitate out of suspension and adsorb to the rock or pipe surfaces creating production problems for producers by reducing production rates and increasing the possibility of expensive mechanical failure. In addition to asphaltenes, other undesirable downhole products form such as scale, paraffins, fines, pipe dope, sulfur, heavy oil tar by-products and water blocks. Such accumulated deposits affect oil well productivity. Remediation treatment fluids are further typically used to remove such undesired deposits prior to the introduction of stimulation fluids. The chemical additives serve to disperse and break-up solid components within the drilling fluid. In addition, they serve to decrease surface activity between the two fluids.
In the past, attempts have been made to remove the paraffin and/or asphaltene by mechanically cutting it out of the well or using a so-called “hot-oiling” technique. The cutting procedure is a relatively crude procedure and requires extensive well shutdown time. Moreover, it is extremely inefficient inasmuch as substantial amounts of paraffin and/or asphaltene remain in the well. This remaining paraffin and/or asphaltene provides seed crystals which promote the rapid formation of additional paraffin and/or asphaltene. Consequently, paraffin and/or asphaltene builds up and the blocking situation quickly occurs again.
In the hot-oiling method, produced crude is heated to a temperature well above the melting point of the paraffin and/or asphaltene and is then circulated down through the annulus of the well and returned to a hot-oil heating truck via the production tubing. The purpose here is for the hot oil to melt and/or dissolve the paraffin so that it can be removed from the well in liquid form. This is an expensive method since the crude must be put through a heater treater along with a demulsifier in order to facilitate the removal of solids and water therefrom. In this method, the crude oil used is taken from the stock tank and has thus already made one pass through the treating facility and has already been demulsified. Another disadvantage to this method is that in many instances, 100% of the fluid injected is not recovered and thus some is lost to the reservoir.
During the hot-oiling process, a paraffin dispersant which is based on a petroleum sulfonate is added to the crude as it is being heated. The paraffin dispersant assists in dispersing the melted paraffin in the hot-oil phase.
Moreover, this technique is very dangerous, particularly with wells producing a crude having a low flash point. Indeed, such wells cannot be hot-oiled because the auto-ignition temperature of the oil is so low. Thus, bringing the oil in direct contact with a heating mechanism creates a substantial fire hazard.
Another procedure that has been tried is the so-called “hot acid” technique. In this process, an attempt is made to melt paraffin using a combination of hot water, heated xylene and hot acid. However, while this mixture may have some effect on the removal of carbonate scale build-up in wells, it has not presented a satisfactory answer to paraffin and/or asphaltene removal.
An additional disadvantage of each of the above methods is due to the fact that normally it is desirable to subject a well to acidization at some subsequent point in time after the paraffin and/or asphaltene removal. The mechanical cutting technique, hot-acid technique, and the hot-oil techniques leave the well bore, the area surrounding the well bore casing and tubing “oil wet”. This is a disadvantageous situation for subsequent acidization. In the acidization technique, a mineral acid solution is introduced into the well to remove mineral deposits. The acid solution is aqueous and, if the interior portions and mechanical elements of the well are oil wet, direct contact of these surfaces with the acid solution is inhibited making the acidization treatment much less effective. Such stimulation of oil and gas wells is a well known process and is described in U.S. Pat. No. 4,541,483.
Other more recent prior art attempts have been made trying to overcome the presence of the undesirable paraffin and/or asphaltene and scale in oil wells and associated equipment as stated below. In U.S. Pat. No. 3,930,539 there is disclosed a method for increasing the production in wells by the utilization of hydrochloric and phosphoric acid followed by ammonia to create a violent exothermic reaction at the bottom of the well and thus disintegrate the limestone and emulsify the paraffin thereby creating larger passages in the formation and which permits greater flow. This is not desirable since the reactions themselves create safety hazards and the paraffin still remains.
In U.S. Pat. No. 4,836,286, there is disclosed a method of removing flow-restricting matter such as paraffins from wells by use of a three stage process of introducing various solvent solutions into the bottom of the well over a period of time and then removing the solvents there from and passing an electrical charge there through followed by the reintroduction into the well. This has the disadvantage of numerous steps and the use of electrical charge.
In U.S. Pat. No. 6,593,279, there is disclosed the use of an acid based emulsion for cleaning oil sludges from well cuttings, well formations and down hole wells. These emulsions contain water, a surfactant mixture, an oil, a solvent, and an oxidizer. There is no disclosure of the removal of scale.
In U.S. Pat. No. 4,278,129, there is disclosed a two stage process of stimulating an oil well by the use of an oxyalkylated phosphate ester surfactant followed by the introduction of a hydrocarbon to drive the ester into the formation.
In U.S. Pat. No. 4,813,482, there is disclosed a method of removing paraffin from an oil well by treating the well with a heated solution containing a surfactant, a hydrocarbon solvent, and water in order to disperse the paraffin with the well into the solution.
In U.S. Pat. No. 5,909,774, there is disclosed a method of cleaning up a producing interval of a well bore drilled using a synthetic oil-water emulsion drill-in fluid. This method involves the use of three treatment fluids in three separate stages.
In U.S. Pat. No. 6,112,814, there is disclosed a method for cleaning a well bore plugged with deposits of heavy hydrocarbons and finely-divided inorganic solids by circulating a surfactant composition containing an alkyl polyglycoside, an ethoxylated alcohol, a caustic and an alkyl alcohol through the well bore with a coiled tubing.
In U.S. Pat. No. 7,296,627 there is disclosed a process for the simultaneous removal of asphaltene, paraffin and scale using an aqueous cleaning emulsion comprising of water, hydrocarbon solvent, detergent and mineral acid.
Well treatment fluids presently used for removing unwanted deposits drilling of hydrocarbons such as asphaltenes and paraffins from the wellbore, are either not biodegradable or are less efficacious than desired. Typically, producers perform systematic removal treatments to overcome the effects of asphaltene deposits. Historically, xylene has been used to remove these organic deposits; however, xylene does not change the wettability of the rock surface, resulting in treatment effectiveness that is often short-lived. Further, xylene mixtures have a low flash point (77-84° F.) and contain objectionable components such as benzene, ethyl benzene and toluene (BETX). There is a continued need for more effective methods and systems for enhancement of oil recovery, wellbore remediation and formation stimulation. In particular, there is a need for new systems that are biodegradable.
Nanoemulsions have been discovered to be useful to the oil field. More particularly water-in-oil (W/O), oil-in-water (O/W) and other classes of nanoemulsions have found beneficial application in drilling, completion, well remediation and other oil and gas industry related operations. Additionally, nanoemulsions may reduce friction pressure losses, as well as reduce subsidence of solid weight material during oil and gas operations. Nanoemulsions have many physical properties that distinguish them from other emulsions. Due to their small mean droplet size, which is often smaller than optical wavelengths of the visible spectrum (thus less than about 400 nm), nanoemulsions usually appear transparent or translucent to the naked eye, even at high droplet volume fractions. The terms sub-micron emulsion (SME) and mini emulsion are sometimes used as synonyms for the term nanoemulsion. Nanoemulsions have great potential for use as cleaning agents. Nanoemulsions can typically be formulated using less surfactant than is required for many micro emulsions.
A nanoemulsion may be defined as a type of emulsion wherein the dispersed/discontinuous phase has a mean droplet size of less than 1000 nm; the components of the continuous and dispersed/discontinuous phases must be immiscible enough to allow for the respective phase formation. Nanoemulsions are typically prepared by imparting sufficient shear to reduce the droplet size of the immiscible internal phase below 1000 nm. This may involve the use of a high speed mixer, a high pressure homogenizer, a high frequency ultrasonic device or a small pore membrane. Nanoemulsions are prepared by providing an external energy input to the oil/water/surfactant system using high shear stress or inertial disruption to overcome the effect of interfacial tension and reach the levels of Laplace pressure of droplets having the expected size to fragment large micro scale droplets into the nanoscale. An excess of surfactant may be present in the continuous phase in the form of micelles that dissociate into monomers that rapidly adsorb on the newly created surface area of the nanoemulsion sized droplets and coat the interfacial film, thereby preventing shear-induced coalescence. The concentration of surfactant in the system also plays a role in determining the limiting droplet size when all other parameters are fixed. Apparatus suitable for preparing the nanoemulsions by mechanical energy input include, but are not limited to, devices offering a high power density with a small and well defined disruption zone.
Asphaltenes are aromatic-based hydrocarbons of amorphous structure. They are present in crude oils in the form of colloidally dispersed particles. The central part of the asphaltene micelle consists of high molecular weight compounds surrounded and peptized by lower weight neutral resins (maltenes) and aromatic hydrocarbons). Asphaltenes are lyophobic with respect to low molecular weight paraffinic hydrocarbons and lyophillic with respect to aromatics and resins. Asphaltene deposits in near wellbore subterranean formations, in well tubing and perforations, and in transfer lines, storage tanks, surface equipment, and pipelines hinder production and transport of high asphaltenic crudes from wells. Various compositions and treatments have been proposed and used for removing these deposits, but new treatments are needed; especially treatments which are non-reactive chemically with subterranean formations, environmentally friendly, inexpensive, easy to use, and effective.